The process of signal transduction across the lymphocyte plasma membrane is under investigation. Novel fluorescent probes of physiologic parameters such as membrane potential, intracellular pH and intracellular calcium, in conjunction with the high sensitivity and single cell resolution of flow cytometry, are being utilized. We have found that positively charged cyanine dyes are not suitable for measuring membrane potential in cells containing mitochondria, but that negatively charged oxonol dyes are reliable indicators. Lymphocytes and monocytes, but not granulocytes, buffer membrane potential over more than the physiologic range of extracellular potassium ion. T lymphocytes, but not B lymphocytes, possess a calmodulin-dependent, calcium-sensitive potassium channel. A new calcium probe, indo-1, is very useful for measuring intracellular free calcium ion. We find that blocking the IgG Fc receptor of B cells significantly prolongs the calcium transient induced by anti-immunoglobulin.